The next time you're designing the backend for a high-traffic site, I think you should give it another look. As jbert pointed out, the MySQL query cache is empt

The next time you're designing the backend for a high-traffic site, I think you should give it another look. As jbert pointed out, the MySQL query cache is emptied every time you make any change to the table, making it pretty useless unless you only ever do selects. And even for the selects, wouldn't you rather select by key from a hashtable in O(1) than select from a btree index in O(log n) at best?